The reflectance of strong absorption band in condition of cirrus atmosphere is simulated by the general atmospheric radiation transmission software. The effects of cirrus particle’s shape and effective size, optical thickness and cirrus altitude on atmospheric top reflectance and background radiation are analyzed. A radiation intensity calculation model of high-temperature gas target is established and the spectral radiation characteristics of target radiation transmitted to the top of atmosphere in cirrus atmosphere are obtained by simulation. The target background contrast at the top of atmosphere is computed, and the influences of cirrus characteristic parameters and target altitude on target background contrast are also discussed. Results show that cirrus atmospheric reflectance increases with the increasing of cirrus altitude and optical thickness, and decreases with the increasing of cirrus particle effective size in the strong absorption band of vapor. The target background contrast is greatly affected by cirrus parameters. The presence of cirrus has a big disturbance to target observation, especially when the target altitude is below 7 km.